Such groupings are otherwise unknown among living and extinct arthropods (the group that includes crustaceans, insects, and spiders), the study team said.

The only obvious living parallels for this kind of setup are salps, jellyfishlike ocean-drifters that are related to sea squirts, the researchers said.

Salps form lines of interlinked colonies as part of their reproductive cycle.

"But we don't find this type of reproductive mode in other arthropods," Siveter said. "So then we come back to a more simple explanation, that these animals congregated in this fashion for migration."

A loose analogy exists in today's spiny lobsters, which are found in tropical regions including the Caribbean, he said.

Though the lobsters don't link up in a chain during migration, "they march roughly in lines on the seabed at certain times of the year," Siveter said.

Migration does seem the most rational explanation for the fossils, agreed Strausfeld, of the University of Arizona.

"That's all you have to go on, quite frankly, based on the crustaceans tramping around on the planet today," he said.

Another possibility, Strausfeld said, is that the animals "may have been preserved before they hatched out of this long egg case."

The strength and precision of the chains, with their repeated head and tail insertions, suggests some kind of packaging was involved, he said.

"The egg case, if it's gelatinous, wouldn't be preserved," he added.

And the bigger, unattached specimen could represent an adult or a hatchling.

"Maybe the sac breaks down when the animal reaches a certain size," Strausfeld said.

Evolutionary Test?

The fossils date to the so-called Cambrian Explosion, a rapid evolutionary flowering when many of the major animal groups appeared.

"At that time in history there were some very strange things, and a lot of experimentation going on," Strausfeld said.

Although he knows no precedent for crustaceanlike animals producing egg cases that house numerous developing embryos, "it's possible [the fossils represent] strange ways of reproduction and development that are no longer with us," he added.

Siveter, of the University of Oxford, said the new discovery "gives us another window on this early proliferation of life, when all the major groups which sustain biodiversity to the present day were introduced to the fossil record."